The present invention relates to a device for detecting abnormality of air pressure and temperature in a tire during running of a vehicle.
Conventionally, a tire air pressure warning device is known for warning of a reduction in air pressure in the tire, to avoid danger caused by damage of the tire due to abnormal heat generation in the tire under a reduced air pressure. An example of the tire air pressure warning device is shown in FIGS. 8 and 9 (Japanese Utility Model Laid-open Publication No. 55-10159).
Referring to FIG. 8, reference numerals 21 and 22 designate a tire and a road wheel, respectively. A warning device 23 is fixedly mounted at a central portion of the road wheel 22 by tightening a wheel nut 24 to a wheel bolt. The warning device includes a battery, a buzzer driving circuit, a tire rotation detecting switch, etc. An air pressure sensor 25 is mounted to a wheel rim to receive an air pressure from an air valve and detect a reduction in the air pressure in the tire. A battery check button 26 is provided at the air pressure sensor 25.
Referring to FIG. 9 which shows a buzzer driving circuit in the warning device 23, the circuit includes a battery 27, a buzzer 28 and a tire rotation detecting switch 29 adapted to be closed during rotation of the tire and be opened upon stoppage of the tire. The air pressure sensor 25 includes an air pressure sensing contact 25a and a battery checking contact 25b. A transistor Tr1 is provided to control the supply of current to the buzzer 28. The transistor Tr1 goes on when a signal is input thereto from an oscillating circuit 30 such as an astable multivibrator, and connects the buzzer 28 through the air pressure sensing contact 25a to the battery 27. The tire rotation detecting switch 29 inputs a signal to the oscillating circuit 30 when the air pressure sensing contact 25a is closed upon reduction in air pressure during rotation of the tire, and thereby turn on the transistor Tr1 to drive the buzzer 28. On the other hand, when the tire 21 is in a rest condition, the tire rotation detecting switch 29 does not input a signal to the oscillating circuit 30 to maintain the transistor Tr1 in an off state and thereby prevent current consumption of the battery 27. The battery checking contact 25b is closed by depressing the check button 26 of the air pressure sensor 25 to input a signal to the oscillating circuit 30 and thereby enable an operator to confirm whether or not the battery 27 is consumed. A capacitor C1 and a resistor R1 form a hold circuit for operating the oscillating circuit 30 for a period of time depending on the resistor R1 once the capacitor C1 is charged by closing the battery checking contact 25b or the air pressure sensing contact 25a and the tire rotation detecting switch 29.
As mentioned above, a reduction in air pressure of the tire is detected by closing the air pressure sensing contact of the air pressure sensor in the air pressure warning device. Further, as the air pressure sensor is connected through a lead wire to the warning device, there is a possibility of the lead wire being broken. Accordingly, a long time is required for maintenance and inspection of the warning device.
Furthermore, the warning device as mentioned above detects a reduction in air pressure only, and cannot detect abnormal temperature increase without a change in air pressure. To solve this problem, it is necessary to provide a device for detecting an air temperature in the tire and a circuit for processing the result of detection of the air temperature in addition to the tire air pressure warning device, which will cause an increase in the number of parts, an increase in weight of the device and an increase in cost.